Refractory insulating materials for use in high temperature applications (500.degree.-3000.degree. F.) have undergone various stages of development in recent decades. Initially, asbestos was a popular insulating material. Due to health and safely concerns, asbestos was substantially replaced with ceramic fibers and other materials. Ceramic fibers are customarily used today in applications involving the iron and steel industry, and other applications.
Ceramic fibers represented an expensive replacement for asbestos because the installation of ceramic fibers is expensive and labor-intensive. Also, ceramic fibers pose their own health and safety hazards in that gases which are evolved inside the furnace or other high temperature structure can sometimes pass through the layer or layers of ceramic fiber insulation. For example, corrosive gases can pass through the insulation and cause damage to the steel shell of the high temperature structure. Where a steel shell is damaged or is not present, toxic gases generated in a furnace or other structure can penetrate the ceramic fibers and affect persons standing nearby.
In order to be effective as an insulator, a material should have voids or pockets which contain air or another (e.g. inert) gas having a low thermal conductivity. In order for the same material to be impermeable, it is desirable that the air pockets be substantially closed, walled or separated from each other, so that gaseous materials cannot pass through the insulating material. In order for the same material to be relatively inexpensive, it is desirable that the material lend itself to installation using continuous, automated or semi-automated equipment.
Ceramic fibers meet only the first of the three criteria listed above. There is a need in the iron and steel industry, and in other industries, for a refractory insulating material that not only has a low density (i.e. high void volume), but which is impermeable and easy to install.
Pumpable refractory materials (i.e. castable refractory materials which can be continuously installed using a concrete pump or similar device) are known in the iron and steel industry from U.S. Pat. No. 5,147,830, issued to Banerjee et al. Until now, however, this highly efficient technology has not been applied to refractory insulating materials due, in large part, to the absence of a suitable refractory insulating material which can flow through a pump.